Bug 14971: fix RIS export

Bug 14971: fix RIS export

This patch removes a limit in RIS.pm print_isbn sub.

To test:
1) Find in opac a book with an isbn with length(isbn) > 10
(with dashes or isbn13)
2) Export record to RIS format, check truncated SN field
in exported file
3) Apply the patch
4) Export again, no more truncation

Signed-off-by: jvr <jvr@fct.unl.pt>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 906d2108b9054a68168219136d69e6d783f156fb)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>